1. Field of the Invention
The present invention relates to an optical element composed of a ferroelectric single crystal capable of photo-induced refractive index changes and more particularly to an optical element composed of strontium barium niobate single crystal containing a dopant to increase the sensitivity to a beam of light.
2. Brief Description of the Prior Art
Although optical damage effects in response to a laser beam in a ferroelectric crystal is well known and very harmful in the case of an element for a light modulator, a second harmonics generator or a light deflector the effect suggests a new use of such optic crystals for an optical memory element. In general, an optically damaged crystal undergoes a local change in its refractive index at the irradiated portion of the crystal which can maintain the change for some time and has the capability for reverting to the original state by the irradiation of a beam of light of the same wavelength on the entire surface thereof. Optical information can be recorded in a crystalline body as refractive index changes and can be erased by making use of the above-mentioned phenomena. In other words, a reversible optical recording element can be obtained. Furthermore, high diffraction efficiency may be expected because the information can be recorded in the form of a phase-volume hologram by holographic recording technology.
In order to obtain a recording element of such high capabilities theoretical and experimental studies on the optical damage effects have been made and now it is understood that the optical damage effect is caused by the following mechanism.
There are, in crystals undergoing the optical damage effect filled traps caused by impurities, vacancies or the like. The filled traps easily generate free electrons in the conduction band when the electrons in the filled traps absorb energy from the light beam having a wavelength corresponding to the energy gap between the filled traps and the conduction band. Then, the generated free electrons diffuse out into the outer irradiated portion and are trapped by the empty traps caused by impurities, vacancies or the like. As a result, there is generated in the inner and outer irradiated portion a macroscopic space charge field which induces the refractive index changes in the crystal.
As conventional materials developed for optical storage materials having the capability for photo-induced refractive index changes LiNbO.sub.3 doped with iron or a metal element of a rhodium group have been well known. Information can be written in and erased from them by argon laser irradiation, because each of them has an absorption band wavelength of about 4880 A. Furthermore, non-destructive read out can be achieved in response to helium neon laser irradiation.
However, these conventional crystals have very low sensitivities during writing and erasing, and high power irradiation over a long period of time are necessary for performing the functions required in practical memory elements. It is well known that such difficulties have significantly hindered their practical use.
Attempts have been made to increase the writing and erasing sensitivities of strontium barium niobate single crystal by applying an electric field simultaneously with the irradiation. This method is disclosed by U.S. Pat. No. 3,652,145 and requires the application of an electric field for improving the sensitivity. Accordingly, this previous method has difficult problems such as dielectric breakdown and complication of the apparatus.